Cytotoxic effect of lymphocytes from pregnant mice on cultivated tumor cells. I. Specificity, nature of effector cells and blocking by serum.

Lymph-node and spleen cells from pregnant BALB/c mice were found to be cytotoxic (by a microcytotoxicity assay) to cultivated syngeneic tumor cells. Target cells included a series of in vitro transformants derived from BALB/c 3T3 cells (3T3-MSV,3T3-SV40 and 3T12), as well as two lines of tumors induced in vivo. Skin fibroblasts and non-transformed 3T3 cells were not similarily affected. The cytotoxic lymphocyte effect was decreased by treatment with anti 0 serum and complement but was not removed by passage through a column removing cells with immunoglobulin at their surfact. It could be blocked by serum taken from pregnant mice and from mice carrying methylcholanthrene-induced sarcomas. These sarcomas could be different from the ones used as targets in vitro, but the blocking effect was higher when serum was tested from mice carrying the respective tumor studied. The blocking activity of serum from pregnant mice was removed by absorption with neoplastic cells, taken either from culture or directly from mice, and with fresh embryonic cells, but not to the same extent by absorption with explained fibroblasts, 3T3 cells, or adult kidney cells. Absorption was most effective when the same line of cells was used as targets. Both the cytotoxic lymphocyte effect and the blocking serum activity were seen already during the first pregnancy and lasted for at least 1 month subsequent to delivery. Serum from pregnant mice could "arm" control lymph-node cells and produce a lymphocyte-dependent cytotoxic effect on neoplastic, but not on control, target cells.